Acid rainfall is caused by the discharge of sulfur dioxide into the atmosphere by coal fired power plants, internal combustion engines, and various other industrial sources. Such acid rainfall is environmentally problematic in that it will leach metals from pervious surfaces such as lawns and cultivated fields, as well as from impervious surfaces such as metal roofing and parking lot surfaces. In certain instances, these leached, or dissolved metals, may be harmful to particular biota living in surface waters that receive acid stormwater runoff.
Performance data collected by monitoring stormwater discharged from existing zinc coated steel roofing, and data collected for the surrounding environment receiving such stormwater runoff, clearly show that acid rainfall leaches zinc from galvanized and Galvalume® coatings. In response to such findings, various state and local environmental regulatory agencies are beginning to place stringent zinc level criteria on stormwater discharged from zinc coated roof surfaces. For example, the State of Massachusetts and King County in the State of Washington are two regulatory agencies that have passed stringent codes directed to the treatment and disposal of zinc containing stormwater discharge. These new environmental regulations present problems for architects and steel producers alike, especially along the eastern half of the nation where rainfall has a typical pH level of about 4.8 or lower due to increased concentrations of sulfuric and nitric acids in the atmosphere.
One approach for dealing with the zinc-leaching problem is often referred to as “end of pipe” treatment. End of pipe treatment systems are either natural or artificial wetlands that use one or a combination of mechanisms to remove heavy metals from urban runoff (leachate) and mine drainage operations. However, although such wetland treatment systems are found useful for removing zinc ions from wastewaters and drainage streams, they are expensive because they require large land areas to be effective. Such large systems are capital intensive, and they require constant maintenance. Therefore, there is a need within the art to provide a water treatment system that is low cost, that requires minimal or no attention during operation (passive), and is capable of removing dissolved metals so that the treated stormwater runoff meets environmental regulatory standards for zinc concentration levels before it is discharged to environment.